Conventionally skins/hides are stabilised against microbial degradation by tanning. Chrome tanning has been the most predominant method of tannage for the commercial purpose. The major limitation associated with this is that the chrome tanned leathers are empty in nature and hence need heavy retanning for its final performance. Unlike chamois leather, which is essentially of oil tanned, chrome tanned leather does not provide any lubrication among the fibres. Hence strong fatliquoring becomes mandatory for the desired level of softness.
Conventional chrome tanning practices employ basic chromium sulphate salt, a progenitor of several chromium species, having various charges and degree of polymerisation. The kinetic inertness of certain chromium species lends itself to poor exhaustion (50–70%) of chrome tanning salt. Thus the commercial chrome tanning activities release chromium in the range of 2000–5000 ppm.
Bellavere et al (Environmental Technology Letters, 2, 119, 1981), report that the environmental consequences arising from discharge of industrial wastewater containing chromium into water bodies are enormous. Tsou et al (Chemical Research in Toxicology, 10, 962, 1997) have proved that chromium has the ability to cause cancer, Blankenship et al (Toxicology and Applied Pharmacology, 126, 75, 1994) have reported even cell death as ultimate result due to the influence of chromium. Wide ecological concern as well as economic loss has compelled researchers to look for suitable alternatives to chromium.
Several methods have been evolved over the years for better management of chromium in leather industry, as reported by Chandrasekaran (Leather Science, 34, 91, 1987). Among them the development of mineral alternatives to chromium have been gaining momentum.
Aluminum tanning is one of the oldest methods of tanning. An old name ‘tawing’ has been used for this process, which consists of treating the skin with potassium alum in the form of paste including sodium chloride, egg yolk, flour and water. Aluminum tanned leathers are sensitive to water and heat. Aluminum tanned leather has a shrinkage temperature in the range of 75–82° C. depending on the method of tanning used. Its water resistance is unsatisfactory, since even a wash with the cold water slowly removes aluminium compounds from leather. Aluminium being d° element, its complexes are much less stable than those of chromium; and thus their binding to collagen is much weaker. This is the reason why aluminium salts are used in contemporary tanning almost exclusively in combined tannage to processes. Generally aluminium tanning is done in floats of zero basicity at high concentrations in the presence of sodium chloride to prevent swelling, because the pH of the float is as low as 2.5–3.5. No washing is employed after tanning.
A detailed study of the masking action of organic acids on aluminium sulfate and chloride was done by Simoncini et al (Cuoio Pelli Mat. Conc. 54, 439 & 711, 1978). They have described a tanning method based on the use of aluminium complex containing citric acid or ethylenediamine tetra acetic acid as complexing agent, which gives good results. However, the process requires a pretanning and retanning with vegetable tans and glutaraldehyde.
In a detailed study by Williams-Wynn (Journal of the Society of Leather Technologists and Chemists, 53, 64, 1969) using various ligands such as formate, acetate, lactate, tartrate and citrate, most stable leathers are obtained with formate, acetate and lactate with shrinkage temperature in the range of 74–78° C. If the complexing agent is too strong it will not be displaced by protein carboxyl groups and the skin will remain untanned. Similar conclusions have been drawn by Kuntzel and Rizk (Leder, 13, 101, 1962).
Mezey (PhD Thesis, Faculty of Science, Lyon, France, 1925) has studied the tanning effect of aluminum sulphate on unlimed skin at various basicities in the presence and absence of common salt. The absence of salt produces hard and horny translucent skin without leathery feel upon drying, which is due to swelling induced by absorption of sulphuric acid. The presence of a sufficient amount of common salt to repress the swelling effect produces supple and opaque leather. However, on washing it reverts to the swollen unlimed skin condition. Studies on tanning using previously adjusted basic solutions of aluminium sulphate by Chambard and Grail (Bull. assoc. franc. chimistes inds cuir et doc. sci. et tech. ind. cuir, 10(3), 17, 28, 1948) and Mezey (PhD Thesis, Faculty of Science, Lyon, France, 1925) have proved that the tanning using various basic aluminum sulphate solutions without common salt results in hard and horny untanned skins in spite of higher absorption of tanning bath components. In the presence of salt, opaque and supple leather having better resistance to hydrolysis than the leather, which is obtained by tannage using zero basicity and shrinkage temperature being about 65 rather than 47° C.
Aluminium based tanning agents like Lutan B and ATC-21 have not found much commercial acceptability because of the ready reversal of aluminium as aluminium hydroxide at pH values in the range of 5.0–6.0. Lipowski (U.S. Pat. No. 4,443,382, 1984) has developed an aluminium salt of aromatic sulfonic acid condensate for retanning applications. However, the polymeric matrix is made using formaldehyde. The most commercial use of aluminium in leather industry until now has been as a dye adjunct. The high cationic potential of Al(III) makes such salts useful for enhancing the color yield.
It has been reported that polyhydroxy aluminium gels aid the exhaustion of chromium during tanning and function as chrome saver by Ramsami et at (Proceedings of the 22nd LERIG, Madras, 167, 1987). However, the reversibility in binding to leathers has limited the usage of Al(III) salts as chrome saver. Aluraa, an aluminium based synthetic tanning agent, has aluminium in the stabilized form and binds irreversibly to the substrate as reported by Kanthimathi et al (Leather Science, 32, 59, 1985). By using this aluminium syntan as chrome saver, the exhaustion level of chromium improves to about 90% as reported by Rao et al (Proceedings of the XXVth ILTCS congress, Chennai, 295, 1999). However, this syntan was used only as a co-tanning agent along with 4–6% chrome salt. Further, the syntan was based on formaldehyde condensates. Swarna et al (Proceedings of the XXVth IULTCS Congress, Chennai 322, 1999) has reported about such formaldehyde condensates that the tanned leathers on aging may release free formaldehyde ranging from 1500 to 15000 ppm, which does not meet the tolerable limit of 5–10 ppm of free formaldehyde as accounted by Mark et al (Kirk Othmer Encyclopedia of Chemical Technology, Volume 13, John Wiley and Sons, New York; 3rd Edition, 1978). The tanning potency of phenol-formaldehyde condensates has been reported by Gustavson (The Chemistry of Tanning Processes, Academic press, Volume 2, p. 4, 1956). But the major disadvantages of such polymeric condensates are    a) the tanned leathers show discoloration    b) the sulfonic acid groups introduced in the system to give soluble matrix would competitively inhibit the binding of anionic dyes    c) slow release of formaldehyde from the tanned leathers on aging may pose hazardous environment.
The main pre-requisite for conventional chrome tanning process, carried out using either basic chromium sulphate (BCS) or high exhaust chrome tanning salt, is to pickle the pelt by employing acid and salt in aqueous medium whereby the emanated liquor is loaded with dissolved solids, chlorides and sulphates, leading thereby to the environmental pollution.